Brake system

ABSTRACT

A vehicle brake system is described in which a lower brake pressure is fed in at the rear axle compared with the brake pressure of the front wheels. For the purpose of feeding in this brake pressure, the difference VD between the fastest front wheel speed and the slowest rear wheel speed is formed and compared with a threshold value which depends on the vehicle deceleration. An increase in pressure is undertaken until the threshold value is reached. The threshold value is, furthermore, larger in the case of travel in a straight line than it is in the case of travel in a curve.

PRIOR ART

Brake systems are known in which, by means of pulsed valves, a brakepressure is fed in at the rear wheels of a vehicle which, when comparedwith the front axle brake pressure, corresponds to that of the knownideal brake pressure distribution (DE 39 01 923).

SUMMARY OF THE INVENTION

In the invention, the rear axle brake pressure is fed in as a functionof the motion behavior of the vehicle and independent of the installedbraking force distribution. The system further distinguishes betweenbraking in a curve and braking during travel in a straight line, and anincreased slip, which effects stronger braking by the rear wheels, ispermitted in the case of braking during travel in a straight line.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE is a diagram of the system of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Four speed sensors on the wheels generate wheel speed signals which aresupplied to a block 1 which, from the speed signals of the left-handfront wheel V_(VL), of the right-hand front wheel V_(VR), of theleft-hand rear wheel V_(HL) and of the right-hand rear wheel V_(HR),generates signals

    VXU=(V.sub.VL -V.sub.HR)-(V.sub.VR -V.sub.HL) and

    V.sub.D =V.sub.Vmax -V.sub.Hmin

where V_(Vmax) is the signal of the fastest front wheel and V_(Hmin) isthe signal of the slowest rear wheel. The signal VXU is filtered in afilter 2, the time constant of the filter being controllable by means ofthe vehicle deceleration F and, specifically, in such a way that thetime constant decreases when the vehicle deceleration increases. It isassumed here that the vehicle is equipped with an anti-lock brake systemand that the vehicle deceleration is obtained in an anti-lock brakesystem, from the gradient of the reference speed for example. Thefiltered signal VXG is supplied to a comparator 3 which compares thissignal with a threshold value VXG_(S), which is specific to the vehicle,is supplied via a terminal 3' and is, for example, 2 km/h. Thecomparator 3 emits a signal SAF when this threshold value is notexceeded. The signal emitted indicates that travel in a straight line ispresent. The signal can also be supplied to the anti-lock brake system 7so that it can be taken into account in the anti-lock brake systemcontrol algorithm.

The deceleration signal of the anti-lock brake system 7 is also suppliedto a multiplier 4 which multiplies the deceleration signal once by avehicle-specific value (input via terminal 4') and, furthermore,undertakes a weighting of the product as a function of whether thesignal SAF for travel in a straight line is present (weighting a₁) ornot (weighting a₂), the weighting factor a₁ being greater than a₂ (e.g.a₁ =2 and a₂ =1). The output signal of the multiplier 4 obtained in thisway represents the comparison value VD_(S) for the signal VD. If thesignal VD exceeds the signal value VD_(S), a comparator 5 generates asignal by means of which a valve 6 is activated which then permits nofurther increase in brake pressure at the rear axle. In the case ofindividual wheel control at the rear axle, the inlet valves of the tworear wheel brakes can replace the valve 6. In the case of travel in astraight line, the valve 6 (or the valves) are at first activated at ahigher slip than in the case of travel in a curve.

We claim:
 1. Method for controlling brake pressure in a motor vehiclehaving front wheels, rear wheels, and brakes at said wheels, said methodcomprisingmeasuring the speeds and V_(VL) and V_(VR) of the frontwheels, determining the fastest front wheel speed V_(Vmax) from V_(VL)and V_(VR), measuring the speeds V_(HL) and V_(HR) of the rear wheels,determining the slowest rear wheel speed V_(Hmin) from V_(HL) andV_(HR), determining a difference V_(D) =V_(Vmax) -V_(Hmin), determiningwhether the vehicle is travelling in a straight line or in a curve,determining a vehicle deceleration, determining a comparison valueV_(DS) based on said vehicle deceleration and whether said vehicle istravelling in a straight line or a curve, said comparison value V_(DS)being larger when said vehicle is travelling in a straight line thanwhen said vehicle is travelling in a curve, comparing said differencev_(D) to said comparison value V_(DS), and preventing an increase inbrake pressure at said rear wheels when v_(D) exceeds V_(DS).
 2. Methodas in claim 1 wherein travel in a straight line or a curve is determinedbydetermining a wheel speed difference VXU=(V_(VL) -V_(HR))-(V_(VR)-V_(HL)), comparing VXU to a threshold value, and generating a signalindicating travel in a straight line when VXU does not exceed thethreshold value.
 3. Method as in claim 2 wherein said comparison valueis weighted by a first factor a₁ when said signal indicating travel in astraight line is generated and a second factor a₂ when said signal isnot generated, where a₁ >a₂.
 4. Method as in claim 1 wherein VXU isfiltered using a time constant dependent on said vehicle deceleration.